Producing a large image over a large field-of-view generally requires more image data and light output than can be provided by a single projector. Consequently, it has long been standard practice to use multiple projectors to form large scale composite images.
A problem facing designers of multiple projector systems is how to ensure that the composite image appears as a single continuous image and not as a mosaic. There are essentially two main methods of forming a composite image, namely edge-matching and overlapping. Edge-matching involves masking the image to form an image with a hard-edged shape. The hard-edged image is then abutted to an adjoining image. However, it has proved very difficult in practice to achieve an unobtrusive join between adjacent images, particularly when the projection surface is double curved such that the shapes of each projected image are non-orthogonal. Frequently edged-matched composite images contain some bright areas of overlap and/or other dark areas with no image. Mainly for these reasons, edge-matched techniques have been largely abandoned in recent years and have been superseded by overlapping image methods in which adjoining images are deliberately overlapped by an area equal to about 10 to 25% of the width of the image. The brightness of each sub-image is then made to taper off throughout the overlap region so that the sum of the brightness of the overlapping images is nearly constant.
Overlapping image tiling methods are well established but there have nevertheless been many problems with such methods. A particular problem has been in the controlling the uniformity of the image across the overlap regions. When the projectors are set to dark field there is still some residual light output. In the overlap regions this unwanted illumination is provided by more than one projector so the corresponding dark field light level is multiplied. Uniformity can be restored by increasing the light levels in other areas of the image but this has the disadvantage of reducing the contrast of the image.
Optical shadow masks have therefore been used to suppress black levels in the overlap region. The masks can either be manufactured in a fixed configuration or they can be designed to be adjustable so that the necessary profile can be imparted to the mask during installation. A problem with manufacturing the masks in a fixed configuration is that once installed, the masks cannot thereafter be adjusted to compensate for any movement in the projection surface or projection equipment. In particular, if the initial measurements used as the basis for the manufacture of the masks are insufficiently accurate, or there is movement in the surface onto which the images are to be projected, or if the projection surfaces are uneven or are of variable curvature, the resulting mask may give very poor results.
A preferred approach is therefore to manufacture an adjustable curve which is then calibrated in situ. This approach requires the mask to be made from a material which is flexible enough to be formed into the correct shape, but is thereafter stable enough to maintain that shape. Previous approaches have made use of flexible materials including polymers and polymer foams but these materials are known to exhibit creep when left under load for protracted periods. In addition they are typically subject to expansion and contraction as a consequence of thermal cycling due to heat from the projector and diurnal effects. A further problem is that some installations are vulnerable to interference by customers or other personnel who may inadvertently dislodge the masks.
Canadian patent application CA 2227920 (Chun-shan Institute of Science and Technology) discloses a projection system making use of multiple projectors in which a frame holding a plurality of sharp-edged sliders is mounted in front of each projector lens. Diffraction of light at the sharp edges of the sliders is said to enable the brightness of overlapping images from adjacent projectors to be adjusted in order to improve overall picture quality. However, the device disclosed in CA 2227920 would appear to suffer from a number of disadvantages and is unlikely to be suitable for use with modern digital projection apparatus. Because the sharp edges are integrally formed with the sliders, they are of necessity somewhat large. As a consequence, the rather coarse profile created by the sharp edges will provide only a very crude means for adjusting brightness levels in the areas of overlap between adjacent images. Furthermore, the arrangement of the sliders in the Chun-shan device would most cause shadowing and light leakage between the sliders, leading to further loss of resolution of the images in the area of overlap. A further drawback to the device disclosed in CA 2227920 is that, so far as it is possible to tell, the device is only capable of manual adjustment and the arrangement of the frame and sliders does not readily lend itself to automation.